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REPRESENTING A STRUCTURE OF A BODY REGION BY DIGITAL SUBTRACTION
ANGIOGRAPHY

Abstract

A method is provided for representing a first structure of a body region
by digital subtraction angiography. The method includes: receiving a
filler image of the body region created by an angiography apparatus,
which represents a second structure of the body region and the first
structure with a first contrast medium concentration in the first
structure; determining a mask image of the body region representing the
second structure; determining a subtraction image by editing out of the
second structure from the filler image by the mask image; determining a
guidance image representing the first structure based on the subtraction
image; reducing image noise of the subtraction image by the guidance
image; and representing the first structure based on the noise-reduced
subtraction image.

1. A method for representing a first structure of a body region by
digital subtraction angiography, the method comprising: receiving at
least one filler image of the body region created by an angiography
apparatus, wherein the at least one filler image represents a second
structure of the body region and the first structure with a first
contrast medium concentration in the first structure; determining a mask
image of the body region representing the second structure; determining
at least one subtraction image by editing out of the second structure
from the at least one filler image by the mask image; determining a
guidance image representing the first structure based on the at least one
subtraction image; reducing image noise of the at least one subtraction
image by the guidance image; and representing the first structure based
on the at least one noise-reduced subtraction image.

2. The method of claim 1, wherein at least two filler images are
received, which respectively represent the second structure of the body
region and the first structure for a different contrast medium
concentration in the first structure, wherein at least two subtraction
images are determined by editing out the second structure from the at
least two filler images by the mask image, wherein the guidance image is
determined based on the at least two subtraction images, wherein the
image noise of the at least two subtraction images is reduced by the
guidance image, and wherein the first structure is represented based on
the at least two noise-reduced subtraction images.

3. The method of claim 2, further comprising: determining a timing curve
of the contrast medium concentration in the first structure of the body
region based on the at least two noise-reduced subtraction images.

4. The method of claim 3, further comprising: smoothing the timing curve
of the contrast medium concentration by a Savitzky-Golay filter.

5. The method of claim 1, wherein the receiving comprises receiving an
empty image created by the angiography apparatus without a contrast
medium in the first structure, and wherein the mask image is determined
based on the empty image and the at least one filler image.

6. The method of claim 5, wherein, in the determining of the mask image,
the empty image and the at least one filler image are averaged over time
by a weighting function dependent on the empty image.

7. The method of claim 2, wherein the at least two subtraction images are
smoothed, and wherein the guidance image is determined from the at least
two, smoothed subtraction images.

8. The method of claim 7, wherein the at least two subtraction images are
smoothed by a Savitzky-Golay filter.

9. The method of claim 1, wherein the image noise in the at least one
subtraction image is reduced by bilateral filtering depending on the
guidance image.

10. The method of claim 9, wherein picture element values for the at
least one noise-reduced subtraction image are determined in the bilateral
filtering, taking into account picture element values of the guidance
image and taking into account picture element values of the at least one
subtraction image.

11. The method of claim 1, wherein the guidance image is determined based
on the at least one subtraction image by maximum intensity projection.

12. The method of claim 2, wherein respective values for the standard
deviation of picture element values of the at least two subtraction
images corresponding to one another are determined, wherein the values of
the standard deviation are predetermined as respective picture element
values for a scatter image, and wherein the guidance image is determined
as a function of the scatter image.

13. The method of claim 12, wherein a noise-scatter image characterizing
the image noise is determined, and wherein the guidance image is
determined by editing out the image noise from the scatter image by the
noise-scatter image.

14. The method of claim 13, wherein respective values for the standard
deviation of picture element values corresponding to one another of at
least two empty images created by the angiography apparatus with contrast
medium in the first structure, and wherein the values of the standard
deviation are predetermined as respective picture element values for the
noise-scatter image.

15. The method of claim 14, wherein the noise-scatter image is smoothed
by bilateral filtering by a filter kernel dependent on the mask image.

16. The method of claim 13, wherein a regression function is
predetermined and the noise-scatter image is determined by the regression
function from the mask image.

17. The method of claim 16, wherein the regression function is
predetermined based on the reference images of a reference body region
without a contrast medium created by the angiography apparatus.

18. The method of claim 1, further comprising, before the determining of
the at least one subtraction image: compensating for a deviation between
at least two images created by the angiography apparatus caused by a
movement of the body region.

19. An evaluation device configured to represent a first structure of a
body region by digital subtraction angiography, wherein the evaluation
device is configured to: receive at least one filler image of the body
region created by an angiography apparatus, wherein the at least one
filler image represents a second structure of the body region and the
first structure with a first contrast medium concentration in the first
structure; determine a mask image of the body region representing the
second structure; determine at least one subtraction image by editing out
of the second structure from the at least one filler image by the mask
image; determine a guidance image representing the first structure based
on the at least one subtraction image; reduce image noise of the at least
one subtraction image by the guidance image; and represent the first
structure based on the at least one noise-reduced subtraction image.

20. An angiography system comprising: an angiography apparatus configured
to create images of a body region with an evaluation device, wherein the
evaluation device is configured to: receive at least one filler image of
the body region created by an angiography apparatus, wherein the at least
one filler image represents a second structure of the body region and the
first structure with a first contrast medium concentration in the first
structure; determine a mask image of the body region representing the
second structure; determine at least one subtraction image by editing out
of the second structure from the at least one filler image by the mask
image; determine a guidance image representing the first structure based
on the at least one subtraction image; reduce image noise of the at least
one subtraction image by the guidance image; and represent the first
structure based on the at least one noise-reduced subtraction image.

Description

[0001] This application claims the benefit of DE 10 2015 224 806.2, filed
Dec. 10, 2015, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The disclosure relates to a method for representing a first
structure of a body region by digital subtraction angiography. The
disclosure also relates to an evaluation device for representing the
first structure and to an angiography system.

BACKGROUND

[0003] Digital Subtraction Angiography (DSA) is a procedure known from the
prior art for visualizing or representing structures in regions of the
body of a person by diagnostic imaging methods, e.g., x-rays. Such
structures to be visualized may be blood vessels in the body region. In
digital subtraction angiography, a number of images following on from one
another in time may be created by an angiography apparatus, e.g., an
x-ray device. During this sequence of recordings, a contrast medium is
injected into the vessel, through which the vessel is made visible and is
mapped in the recorded images. Before the injection of the contrast
medium, a so-called empty image may be recorded in such cases, which
shows structures of the body region, such as bones, differing from the
blood vessels. This empty image is a so-called mask image, by which the
interfering structures differing from the blood vessels may be edited out
in the images recorded with the contrast medium in the blood vessels, the
so-called filler images. To do this, the mask image may be subtracted
from the filler images. The subtraction images resulting therefrom may
only still show the blood vessels filled with the contrast medium. Based
on a timing curve of the contrast medium in the blood vessels, which may
be determined based on the sequence of subtraction images created from
the filler images and the mask image, information may be obtained about a
state of the blood vessel and blocked or constricted vessels, cerebral
aneurysms, or arteriovenous malformation (AVM) may be detected.

[0004] In order to compensate for a movement of the patient, through which
a shift of the structures shown in the images is caused, U.S. Patent
Publication No. 2012/0201439 A1 describes a method for movement
compensation for images, which have been created by digital subtraction
angiography. To do this, a shift vector between a recorded image of the
sequence and a reference image is determined and the recorded image is
corrected based on the shift vector. U.S. Patent Publication No.
2014/0270437 A1 discloses a DSA method in which no empty image or mask
image is needed. Instead, only the filler images are used to create the
subtraction images.

[0005] A DSA sequence may be carried out with a radiation dose of 1.2
.mu.Gray/frame at seven and a half frames per second (fps) with a
duration of around ten seconds. This produces an overall radiation dose
of 90 .mu.Gray. Since this radiation used for imaging may be damaging for
persons, (e.g., for patients, technicians, and medical staff working on
or with the angiography apparatus), it is desirable to keep the radiation
dose as small as possible to reduce the radiation load for the persons in
this way. To do this, low-dosage DSA methods are known from the prior
art. It is problematic here that the quality of the recorded images is
also reduced by the reduced radiation dose, in that the recorded images
exhibit a high image noise, for example. The noisy images make it more
difficult to obtain information about the state of the blood vessels.

SUMMARY AND DESCRIPTION

[0006] The scope of the present disclosure is defined solely by the
appended claims and is not affected to any degree by the statements
within this summary. The present embodiments may obviate one or more of
the drawbacks or limitations in the related art.

[0007] An object of the present disclosure is to provide a solution as to
how the image quality of images created by digital subtraction
angiography may be improved.

[0008] This object is achieved by a method, an evaluation device, and an
angiography system.

[0009] A method serves to represent a first structure of a body region by
digital subtraction angiography. The method includes the following acts:
a) Receiving at least one filler image of the body region created by an
angiography apparatus, which represents a second structure of the body
region and the first structure with a first contrast medium concentration
in the first structure; b) Determining a mask image of the body region
representing the second structure; c) Determining at least one
subtraction image by editing out of the second structure from the at
least one filler image by the mask image; d) Determining a guidance image
representing the first structure based on the at least one subtraction
image; e) Reducing image noise of the at least one subtraction image by
the guidance image and f) Representing the first structure based on the
at least one noise-reduced subtraction image.

[0010] In this case, at least two filler images, which respectively
represent the second structure of the body region and the first structure
for different contrast medium concentrations in the first structure, may
be received in act a). In act c), at least two subtraction images are
determined by editing the second structure out of the at least two filler
images by the mask image. In act d), the guidance image is determined
based on the at least two subtraction images; and in act e), the image
noise of the at least two subtraction images is reduced by the guidance
image. In act f), the first structure is represented based on the at
least two noise-reduced subtraction images.

[0011] The first structure in the body region of a person, (e.g., a blood
vessel), may be represented by the method. To this end, images of the
body region of the person, (e.g., of a head), in which the blood vessel
to be visualized is located, may be created or recorded respectively by
the angiography apparatus of an angiography system. The angiography
apparatus may be an x-ray-based angiography apparatus, which for
recording the images, emits radiation, such as x-ray radiation with a low
radiation dose, (e.g., 0.8 .mu.Gray/frame), to the body region. The
images recorded by the angiography apparatus are provided to an
evaluation device of the angiography system, for example.

[0012] In the method, the evaluation device is provided with the at least
one filler image created by the angiography apparatus, e.g., with a
sequence of filler images recorded after one another in time with
different contrast medium concentrations. The filler image is created by
an image of the body region being recorded by the angiography apparatus,
the first structure of which is filled with an injected contrast medium.
In the recording of the sequence of filler images in this case each of
the filler images may show the same body region with the first structure
for a specific contrast medium concentration. In addition, a second
structure of the body region, (e.g., bone material), is also shown in the
at least one filler image by the angiography apparatus in the recording
of the filler image. This second structure is not to be examined as a
rule and may therefore be removed from the at least one filler image. To
this end, the at least one subtraction image is created, which then only
still shows the first structure represented in the at least one filler
image. In particular, a temporal sequence of subtraction images is
created from the filler images recorded one after another in time.

[0013] The at least one subtraction image is created by the second
structure being edited out by the mask image. The mask image only
represents the second structure. To edit it out, the mask image, after
logarithmization of the mask image and the at least one filler image, may
be removed or subtracted from the at least one filler image. In such a
case, the structure, (e.g., the second structure), which is present or is
shown both in the filler image and in the mask image, is edited out of
the filler image.

[0014] Because of the low dose of radiation emitted by the angiography
apparatus for imaging, the images recorded by the angiography apparatus
exhibit an image noise that worsens the image quality. In other words,
the images are noisy. In order to reduce this image noise in the at least
one subtraction image, (e.g., to de-noise the at least one subtraction
image), the guidance image, which represents the first structure, is
determined. In particular, the guidance image provides contrast
information about the first structure. For de-noising, the at least one
subtraction image may be weighted, for example, with a weighting function
depending on the guidance image or may be locally averaged. This is also
referred to as guided image filtering and corresponds to a guided spatial
smoothing of the at least one subtraction image. For a sequence of a
number of subtraction images, in this case, each subtraction image is
de-noised with one guidance image. By the guidance image, a smoothing of
the subtraction images while retaining edges, (e.g., transitions between
the first structure and a background in the subtraction image), may be
achieved. By the guidance image, the subtraction image is thus smoothed
without any reduction in sharpness of the subtraction image. A
subtraction image de-noised or noise-reduced by the guidance image shows
the first structure with a high contrast. The at least one noise-reduced
subtraction image may be displayed, for example, on a display device of
the angiography system.

[0015] The reduction of the image noise in the subtraction images caused
by the low radiation dose advantageously enables a radiation load on
persons to be reduced and at the same time a high image quality for
analyzing the first structure in the body region to be achieved.

[0016] It proves advantageous for a timing curve of the contrast medium
concentration in the first structure of the body region to be determined
based on the at least two noise-reduced subtraction images. The timing
curve of the contrast medium concentration, which is also referred to as
a time-contrast curve, enables a perfusion of the first structure to be
visualized or mapped. To do this, a first contrast value may be
determined, for example, for a picture element or pixel in the first
noise-reduced subtraction image, which shows a specific volume element of
the body region. In the at least one second noise-reduced subtraction
image, a second contrast value may be determined for the picture element
that shows the same volume element of the body region. The fact that the
subtraction images are created from the filler images recorded after one
another in time thus enables the timing curve of the contrast values and
thus of the contrast medium concentration in the first structure to be
determined. The subtraction images de-noised by the guidance image thus
enable an expressive timing curve of the contrast medium concentration to
be achieved.

[0017] The timing curve of the contrast medium concentration or of the
time-contrast curve may be smoothed by a Savitzky-Golay filter. In other
words, the sequence of the noise-reduced subtraction images is smoothed
in the temporal direction by the Savitzky-Golay Filter (SGF). The
Savitzky-Golay filter enables the image noise to be reduced, e.g., the
time-contrast curve to be smoothed in the temporal direction without
peaks in the curve being flattened out or shifted during the process. In
other words, the characteristic shape of the time-contrast curve is
retained and not changed. Based on this information-preserving smoothing
of the curve, an evaluation of the curve, for example, to assess the
state of the first structure, may be carried out more reliably.

[0018] In accordance with an advantageous form of embodiment of the
method, in act a) an empty image created by the angiography apparatus
without a contrast medium in the first structure is received and in act
b) the mask image is determined based on the empty image and the at least
one filler image. The empty image may be recorded in this case at the
beginning of the recording sequence, before the contrast medium is
injected into the first structure. The fact that there is no contrast
medium in the first structure during the recording of the empty image
means that only the second structure will be mapped on the empty image.
By contrast with the prior art, not just the empty image, which only
shows the second structure, is used as the mask image, but the mask image
is determined from the empty image and the at least one filler image.
Thus, information may be used from all created images, e.g., the empty
image and all filler images. Thus, a noise-reduced mask image may be
determined and thus, in an advantageous manner, image noise may already
be reduced during determination of the subtraction images by the
noise-reduced mask image.

[0019] In a development, for determining the mask image, the empty image
and the at least one filler image are averaged over time by a weighting
function depending on the empty image. In order to determine a picture
element or pixel of the mask image, the corresponding picture elements of
the empty images and the filler images are weighted in this case.
Corresponding picture elements are to be understood here as picture
elements that show the same volume element of the body region. In
particular, the picture elements corresponding to one another have the
same image coordinates in the images.

[0020] Thus, a weighted average of the respective picture element of the
empty image and of the filler images is determined and the respective
picture element value of the mask image is predetermined as the weighted
average. This is also referred to as weighted averaging. The fact that
the empty image and the filler images of the recording sequence have been
recorded after one another in time means that the weighted averaging of
the recording sequence will be carried out in the temporal direction.

[0021] Since the mask image for editing the second structure out of the
filler images may merely show the second structure, the weighting is
undertaken in such a way that picture elements that show the second
structure are provided with a high weight, while picture elements that
show the first structure, for example, are provided with a low weight.
This means, for example, that the picture elements of the empty image
that only show the second structure are included in the average with the
value "1", while picture elements of a filler image that show the first
structure made visible as a result of the contrast medium are included in
the average value with a lower weight. The noise-reduced mask image may
thus be determined in a simple manner.

[0022] In an example, the at least two subtraction images determined in
act c) are smoothed and in act d) the guidance image is determined from
the at least two smoothed subtraction images. In particular, the at least
two subtraction images are smoothed by a Savitzky-Golay filter. The
subtraction images are thus smoothed in the temporal direction before the
determination of the guidance image by the subtraction images. This
enables an improved guidance image to be determined, by which the
subtraction images smoothed in the temporal direction may be spatially
de-noised.

[0023] The image noise in the at least one subtraction image may be
reduced in act e), depending on the guidance image, by bilateral
filtering. In other words, a filter kernel depending on the guidance
image, (e.g., a Gaussian kernel), is predetermined, by which the
subtraction image will be de-noised. During bilateral filtering, in
particular, picture element values for the at least one noise-reduced
subtraction image are determined taking into account picture element
values of the guidance image and taking into account picture element
values of the at least one subtraction image.

[0024] In bilateral filtering, according to the prior art, a picture
element value of a picture element is determined by picture element
values from other picture elements being weighted depending on their
spatial distance and depending on their color distance to the picture
element. For this purpose, a first weighting function may be
predetermined that describes a spatial distance of the picture elements
of the image. The greater the spatial proximity is, the greater is the
weighting. Additionally, a second weighting function may be
predetermined, a so-called edge stop function, which described a
structural distance of the picture elements of the image, e.g., the color
distance. The greater the structural proximity or similarity is, the
greater is the weight. The edge stop function enables it to be prevented
that edges at which the picture elements are spatially although not
structurally close, (e.g., at which the structural distance is small but
the color distance is large), may be smoothed and desharpened. The
application of the bilateral filter, according to the prior art, may
however lead to a loss of detail information in the images.

[0025] Therefore, by contrast with the prior art, for de-noising a
subtraction image, not the color distance of the picture elements in the
subtraction image itself, but the color distance, (e.g., the structural
distance), of picture elements in the guidance image is taken into
account. In other words, the edge stop function is determined depending
on the picture element values of the guidance image. For example,
Gaussian kernels with a bandwidth that is dependent on the image noise
may be used for the weighting functions. This bilateral filtering of an
image as function of another image, (e.g., the bilateral filtering of a
subtraction image as a function of the guidance image), is also referred
to as joint bilateral filtering. In an advantageous manner, this enables
both the image noise to be reduced in a subtraction image and details to
be retained in the subtraction image.

[0026] The guidance image may be determined based on the at least one
subtraction image in act d) by maximum intensity projection. In Maximum
Intensity Projection (MIP), the guidance image is determined from the
temporal sequence of the subtraction images based on an intensity of the
picture elements of the subtraction images. For this purpose, the picture
element of that subtraction image that exhibits the highest intensity may
be selected from the picture elements of the temporal sequence of the
subtraction images corresponding to one another. If only one filler image
has been recorded and thus only one subtraction image has been created,
the guidance image determined by MIP corresponds to the subtraction
image. Through the selection of the picture elements with the highest
intensities from the subtraction images for the guidance image, the
guidance image has a high contrast enhancement and may thus be used well
for de-noising the subtraction images, e.g., for de-noising subtraction
images with low contrast enhancement.

[0027] In an alternative form of embodiment for determining the guidance
image respective values for the standard deviation of picture element
values corresponding to one another of the at least two subtraction
images are determined, the values of the standard deviation are
predetermined as respective picture element values for a scatter image
and in act d) the guidance image is determined as a function of the
scatter image. The image noise in the images created by the angiography
apparatus may be dependent on an attenuation of the radiation. This means
that image regions that show volume elements of the body region, in which
the radiation has penetrated strong bone material, for example, exhibit a
low contrast-to-noise ratio. If the image noise in this case exhibits a
high intensity, the image noise may barely be reduced by determining the
guidance image by maximum intensity projection. Therefore, in accordance
with this form of embodiment, the standard deviation of the corresponding
picture element values of the subtraction images is determined. The
values of the standard deviation in this case are predetermined as
picture element values for the scatter image, based on which the guidance
image is determined. In this case, the scatter image may be predetermined
as the guidance image, which exhibits a reduced image noise compared to
the guidance image determined by maximum intensity projection.

[0028] It proves advantageous for a noise-scatter image characterizing the
image noise to be determined and in act d) for the guidance image to be
determined by editing out the image noise from the scatter image by the
noise-scatter image. The knowledge underlying the disclosure here is that
the image noise in the recording sequence is dependent on an attenuation
of the radiation. This means that picture elements with a higher noise
level have higher values for the standard deviation, although these do
not exhibit any contrast enhancement. Therefore, the spatially varying
image noise, (e.g., the pixel-dependent image noise), is to be removed
from the scatter image. For this purpose, the noise-scatter image is
determined, which describes the spatially varying image noise, and is
taken away or subtracted from the scatter image. The image produced by
the subtraction of the noise-scatter image from the scatter image is
determined as the guidance image, which exhibits a high contrast-to-noise
ratio.

[0029] In such cases, respective values for the standard deviation of
picture element values corresponding to one another may be determined by
at least two empty images created by the angiography apparatus without a
contrast medium in the first structure and the values of the standard
deviation may be predetermined as respective picture element values for
the noise-scatter image. In accordance with this form of embodiment, a
number of empty images are created by the angiography apparatus. The
picture element values of the noise-scatter image are determined here as
the values of the standard deviation of picture elements corresponding to
one another of the at least two empty images.

[0030] It proves advantageous for the noise-scatter image to be smoothed
by the bilateral filtering by a filter kernel dependent on the mask
image. The edge stop function of the bilateral filter is determined here
as a function of the mask image. The filter used here is thus once again
a joint bilateral filter. The smoothed noise-scatter image is subtracted
from the scatter image for determining the guidance image.

[0031] In an alternate form of embodiment for determining the
noise-scatter image, a regression function may be predetermined and the
noise-scatter image may be determined from the mask image by the
regression function. The knowledge underlying the disclosure here is that
the signal-dependent scattering of the image noise, (e.g., in x-ray
images), is directly proportional to a number of photons and may
therefore be approximated directly from the mask image. The scattering is
non-linear because of the processing of the images, for example, as a
result of the application of the Savitzky-Golay filter to the picture
elements. Since however the processing of the images is the same for each
image sequence recorded by the angiography apparatus, the regression
function may be predetermined and then be provided for each recording
sequence to determine the guidance image. To this end, the predetermined
regression function is applied to each picture element of the mask image
of a recording sequence, which has been determined, for example, from the
empty image and at least one filler image of the recording sequence. A
third-order polynomial may be predetermined as the regression function.
This produces the advantage that a number of empty images do not have to
be created in a recording sequence.

[0032] The regression function may be predetermined based on reference
empty images of a body region created by the angiography apparatus
without a contrast medium. This means that the regression function may be
determined once based on the reference empty images for the angiography
apparatus and subsequently be used for all images created by the
angiography apparatus for determining de-noised subtraction images. The
predetermined regression function may be stored in a memory element of
the evaluation device of the angiography system. The method is thus
simply designed.

[0033] An additional act (act g)) may be carried out before act c), in
which deviation caused by a movement of the body region between at least
two images created by the angiography apparatus created is compensated
for. Through the movement of the body region, which is caused by a
movement of the patient, for example, it may occur that a volume element
of the body region has different positions on the image in two images
recorded one after another in time. Thus, picture elements corresponding
to one another in images following on from one another in time may no
longer be assigned uniquely to one another based on their image
coordinates. In order to prevent picture elements of consecutive images
being assigned to one another incorrectly, this movement of the body
region is compensated for. To this end, as described at the start, a
displacement vector to a reference image may be determined for each image
and the image may subsequently be corrected.

[0034] An evaluation device is provided for representing a first structure
of a body region by subtraction angiography, wherein the evaluation
device is designed to carry out the method.

[0035] An angiography system includes an angiography apparatus for
creating images of a body region and the evaluation device. The images
created by the angiography apparatus, for example, the empty image or
images and the filler image or images, are provided for the evaluation
device for carrying out the method and thus for representing the first
structure based on the noise-reduced subtraction images. The angiography
system may also have a display device, on which the noise-reduced
subtraction images may be displayed.

[0036] The forms of embodiment and their advantages presented in relation
to the method apply correspondingly for the evaluation device, and for
the angiography system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The disclosure will now be explained in greater detail based on an
exemplary embodiment, and with reference to the enclosed drawings, in
which:

[0038] FIG. 1 depicts a schematic image of an embodiment.

[0039] FIG. 2 depicts a schematic image of a flow diagram of an
embodiment.

DETAILED DESCRIPTION

[0040] In the exemplary embodiments described herein, the described
components of the form of embodiment each represent individual features
of the disclosure to be considered independently of one another, which
respectively also develop the disclosure independently of one another and
are thus to be seen individually or in combination other than that shown
as an element of the disclosure. Furthermore, the form of embodiment
described here is also able to be expanded by further of the features of
the disclosure already described.

[0041] FIG. 1 depicts a form of embodiment of the angiography system 1.
The angiography system 1 has an angiography apparatus 2, an evaluation
device 3, and a display device 4. The angiography apparatus 2 is designed
to create images and may be an x-ray apparatus, which may have a C-arm 5.
An x-ray source 6 for emitting x-ray radiation may be attached to one end
of the C-arm 5. An x-ray detector 7 for capturing the x-ray radiation
emitted by the x-ray source 6 may be attached to the opposite end of the
C-arm 5. The evaluation device 3, which is designed for processing the
images created by the angiography apparatus 2, may be a processor device,
such as a digital processor or a computer. The display device 4 may be a
screen, for example, which displays the images processed by the
evaluation device 3.

[0042] A body region 8, for example, a head of a patient, may be examined
by the angiography system 1. In this examination, blood vessels may be
examined as a first structure 9 in the body region 8. A second structure
10, (e.g., bone material), is also present, in particular, in the body
region 8 of the patient. To examine the body region 8, a recording
sequence with images I.sub.t recorded after one another in time is
recorded or created by the angiography apparatus 2, of which three images
I.sub.1, I.sub.2, I.sub.3 are shown here. During the recording sequence,
a contrast medium is injected into the first structure 9 of the body
region 8, of which the spreading out in the first structure 9 over time
may be examined. Based on the timing curve of a concentration of the
contrast medium, deductions may be made about a state of the blood
vessels of the body region 8. In this way, vessel constrictions and
vessel blockages may be recognized.

[0043] In this case, a so-called empty image I.sub.1, which represents the
second structure 10, may be created by the angiography apparatus 2. The
first structure 9 is not shown in the empty image I.sub.1 since the empty
image I.sub.1 was recorded without the contrast medium in the first
structure 9. In addition, so-called filler images I.sub.2, I.sub.3 are
recorded one after another in time by the angiography system 2, which
show the first structure 9 for different contrast medium concentrations
as well as the second structure 10. In this case, the first filler image
I.sub.2 may depict the first structure 9 for a first contrast medium
concentration, and the second filler image I.sub.3, recorded after the
first filler image I.sub.2 in time, may depict the first structure 9 for
a second contrast medium concentration. The images I.sub.1, I.sub.2,
I.sub.3 in this case are recorded with a low radiation dose, (e.g., 0.8
.mu.Gray/frame), which causes the images I.sub.1, I.sub.2, I.sub.3 to
have a high image noise. This is visualized in FIG. 1 based on the
cross-hatching in the images I.sub.1, I.sub.2, I.sub.3.

[0044] The images I.sub.1, I.sub.2, I.sub.3 recorded by the angiography
apparatus 2 are provided to the evaluation device 3. The images I.sub.1,
I.sub.2, I.sub.3 are received in a first method act V1 of a method, which
is shown based on the flow diagram in accordance with FIG. 2. In a second
method act V2, the images I.sub.1, I.sub.2, I.sub.3 may be logarithmized.
In a third method act V3, a movement compensation may be carried out in
the images I.sub.1, I.sub.2, I.sub.3. This means that a displacement of
the recorded body region 8 and thus of the structures 9, 10 in the images
I.sub.1, I.sub.2, I.sub.3, which was caused by a movement of the body
region 8 during the recording of the image sequence, is corrected.

[0045] In a fourth method act V4, a so-called mask image M is determined,
which only shows the second structure 10. The mask image M is determined
in this case from the empty image I.sub.1 as well as from the filler
images I.sub.2, I.sub.3. Since the images I.sub.1, I.sub.2, I.sub.3 are
subject to noise, the mask image M is determined as a noise-reduced mask
image, by a respective weighted sum of the picture element values
I.sub.t(x, y) of the images I.sub.1, I.sub.2, I.sub.3 of the recording
sequence being computed as picture element values M(x, y) of the mask
image M:

M ( x , y ) = t = 1 T w t ( x , y ) I t
( x , y ) . ##EQU00001##

[0046] In this case, the image I.sub.t corresponds to the image at the
point t in the recording sequence that includes t=1 . . . T images. The
image I.sub.1 corresponds to the empty image. W.sub.t(x, y) describes a
weighting value with which the picture element value I.sub.t(x, y) of the
image I.sub.t is weighted and which is dependent on the color distance of
the picture element value I.sub.t(x, y) to the picture element value
I.sub.1(x, y) of the empty image I.sub.1. The weighting function W.sub.t
may be specified as the following formula:

[0047] In this case, .sigma..sup.2 corresponds to the estimated average
noise energy for I.sub.t-I.sub.1. Since the noise level is
signal-dependent, a picture element-dependent or pixel dependent noise
energy .sigma.(x, y) may also be predetermined.

[0048] In a fifth method act V5, so-called subtraction images S.sub.t are
determined, of which two subtraction images S.sub.1, S.sub.2 are
represented here and which show the first structure 9 for different
contrast medium concentrations. The subtraction images S.sub.1, S.sub.2
correspond to the filler images I.sub.2, I.sub.3, from which the second
structure 10 has been edited out by the mask image M. To do this the mask
image M is subtracted from the filler images I.sub.2, I.sub.3:

S.sub.t(x,y)=I.sub.t(x,y)-M(x,y)

wherein a subtraction image S.sub.t is the image at the point t in the
recording sequence that has t=1 . . . T subtraction images. These
subtraction images S.sub.1, S.sub.2, which because of the noisy filler
images I.sub.2, I.sub.3 likewise exhibit image noise, may be de-noised by
a guidance image G. The guidance image G is determined in a sixth method
act V6. To determine the guidance image G, the subtraction images
S.sub.1, S.sub.2 may first be de-noised over time. To do this, the
subtraction images S.sub.1, S.sub.2 may be de-noised in the temporal
direction with a Savitzky-Golay filter, so that a temporally de-noised
subtraction image sequence S'.sub.t is produced. The de-noising image G
may be determined from the temporally de-noised subtraction images
S'.sub.t in this case by Maximum Intensity Projection (MIP):

G(x,y)=max.sub.t=1 . . . TS'.sub.t(x,y)

[0049] In this case, the picture element value S'.sub.t(x, y) of that
temporally de-noised subtraction image S'.sub.t that has the highest
intensity, (e.g., the maximum intensity in the image sequence), is
determined as the picture element value G(x, y) of the guidance image G
from the corresponding picture element values S'.sub.t(x, y).
Corresponding picture element values in this case are picture element
values that have the same x- and y-coordinates.

[0050] As an alternative thereto, to determine the de-noising image G, a
standard deviation between the picture element values S.sub.t(x, y) of
the subtraction images S.sub.t may be determined. The values of the
standard deviation are predetermined as picture element values of a
scatter image. In addition, a noise-scatter image may be determined, of
which the picture element values are determined, for example, as the
values of the standard deviation of picture element values I.sub.1(x, y)
of a number of empty images I.sub.1. In this case, the noise-scatter
image is subtracted from the scatter image and thus produces the
de-noising image G.

[0051] In a seventh method act V7, the subtraction images S.sub.1, S.sub.2
are spatially de-noised by the guidance image G. In other words, the
image noise in the subtraction images S.sub.1, S.sub.2 is reduced by the
guidance image G. To do this, each subtraction image S.sub.1, S.sub.2 is
spatially averaged with a filter kernel of a bilateral filter depending
on the guidance image G. By the bilateral filter, to determine the
picture element values S*.sub.t(x, y) of a noise-reduced subtraction
image S*.sub.t, of which the noise-reduced subtraction image S*.sub.1,
S*.sub.2 are shown here, a weighted average value of each picture element
value S.sub.t(x, y) of a subtraction image S.sub.t is determined as a
function of a proximity N of the picture element S.sub.t(x, y) by the
following formulae:

[0052] The weighting function q describes a spatial similarity of picture
elements S.sub.t(x', y'). To this end, the picture elements S.sub.t(x',
y') are weighted based on their spatial distance to one another, e.g.,
the distances between the image coordinates x-x' and y-y'. The weighting
function r describes the structural similarity of the picture elements
S.sub.t(x', y'). To this end, the picture elements S.sub.t(x', y') are
weighted based on a color distance to one another. However, the color
distance G(x, y)-G(x', y') is determined here based on the guidance image
G. By this dependency on the guidance image G, the bilateral filter is
referred to as a so-called joint bilateral filter.

[0053] Gaussian kernels with respective bandwidths .sigma..sup.2.sub.r and
.sigma..sup.2.sub.q may be used for the two weighting functions r, q. The
bandwidth .sigma..sup.2.sub.r may correspond to a noise energy of the
guidance image G. Since the noise density is signal-dependent, spatially
varying bandwidths .sigma.(x, y) may be considered.

[0054] These spatially de-noised subtraction images S*.sub.t may be
temporally de-noised in a act V8 by the Savitzky-Golay filter being
applied to the images S*.sub.t. In other words, a time-contrast curve,
which represents the timing curve of the contrast medium concentration in
the picture elements S*.sub.t(x, y) of the subtraction images S*.sub.t
corresponding to one another, is smoothed. By the one-dimensional
Savitzky-Golay filter, the time-contrast curve is smoothed in the
temporal direction in this case, without peaks being flattened out or
displaced.

[0055] In a ninth method act V9, after the eighth method act V8 has been
carried out, the method acts V6, V7, and V8 may be repeated. In this act,
a new guidance image G may be determined based on the de-noised
subtraction images S*.sub.t, for example, by MIP or by determining a
scatter image and a noise-scatter image. The de-noised subtraction images
S*.sub.t may then be spatially de-noised once again in the seventh method
act V7 by the new guidance image G. In order to avoid a temporal
oversmoothing of the newly de-noised subtraction images S*.sub.t, the
eighth method act V8 may also be skipped. In a tenth method act V10, the
subtraction images S*.sub.t may be displayed on the display device 4 of
the angiography system 1.

[0056] It is to be understood that the elements and features recited in
the appended claims may be combined in different ways to produce new
claims that likewise fall within the scope of the present disclosure.
Thus, whereas the dependent claims appended below depend from only a
single independent or dependent claim, it is to be understood that these
dependent claims may, alternatively, be made to depend in the alternative
from any preceding or following claim, whether independent or dependent,
and that such new combinations are to be understood as forming a part of
the present specification.

[0057] While the present disclosure has been described above by reference
to various embodiments, it may be understood that many changes and
modifications may be made to the described embodiments. It is therefore
intended that the foregoing description be regarded as illustrative
rather than limiting, and that it be understood that all equivalents
and/or combinations of embodiments are intended to be included in this
description.